1. Field of the Invention
The present invention relates to a directional coupler and more particularly relates to a directional coupler that is preferably used in wireless communication devices or other devices that perform communication by using high-frequency signals.
2. Description of the Related Art
A directional coupler described in Japanese Unexamined Patent Application Publication No. 8-237012 is a known example of an existing directional coupler. The directional coupler is formed by stacking a plurality of dielectric layers, on which coil-shaped conductors and ground conductors are formed, on top of one another. Two coil-shaped conductors are provided. One of the coil-shaped conductors constitutes a main line and the other coil-shaped conductor constitutes a sub line. The main line and the sub line are electromagnetically coupled to each other. The coil-shaped conductors are interposed between the ground conductors in a stacking direction. A ground potential is applied to the ground conductors. In the above-described directional coupler, when a high-frequency signal is input to the main line, a high-frequency signal having power proportional to the power of the foregoing high-frequency signal is output from the sub line.
However, there is a drawback with the directional coupler described in Japanese Unexamined Patent Application Publication No. 8-237012, in that the degree of coupling between the main line and the sub line increases as the frequency of a high-frequency signal input to the main line increases (that is, the degree-of-coupling characteristic is not uniform). As a result, even if high-frequency signals having the same power are input to the main line, when the frequencies of the high-frequency signals vary, the power of each of the high-frequency signals output from the sub line varies. Hence, an IC connected to the sub line has to have a function of correcting the power of a high-frequency signal on the basis of the frequency of the high-frequency signal.